Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. When inspecting specular or quasi-specular surfaces such as semiconductor wafers a bright field (BF) modality may be used, both to perform patterned wafer inspection and defect review. In BF inspection systems, collection optics are positioned such that the collection optics capture a substantial portion of the light specularly reflected by the surface under inspection. Viable BF inspection systems require high radiance illumination and a high numerical aperture (NA) to maximize the defect sensitivity of the system.
Current wafer inspection systems typically employ illumination sources of deep ultraviolet (DUV) radiation with wavelengths as short as 260 nanometers with a high numerical aperture (NA). In general, the defect sensitivity of an inspection system is proportional to the wavelength of the illumination light divided by the NA of the objective. Without further improvement in NA, the overall defect sensitivity of current inspection tools is limited by the wavelength of the illumination source.
In the case of broad band imaging tools, the illumination light is typically delivered to the wafer through a catadioptric objective (combination of reflective and refractive optical elements). In the case of narrow band imaging tools, the illumination light is typically delivered to the wafer through transmissive objectives or microscopes. Thus, current BF inspection tools employ optical sub-systems that include refractive optical elements.
In some examples of BF inspection systems, illumination light may provided by an arc lamp. For example, electrode based, relatively high intensity discharge arc lamps are used in inspection systems. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current.
In some other examples, illumination light may provided directly by a laser. One approach has been the harmonic upconversion of longer wavelength sources to shorter wavelengths. However, the average power that can be reliably sustained is typically below one Watt; far below the ten to one hundred watts average power required for high throughput, high resolution BF wafer inspection. In another example, excimer lasers have been developed with higher average power, but the kinetics of excimer lasers at short wavelengths limit these devices to low repetition rates (e.g., several kHz or less). In addition, these lasers are very short pulse lasers (e.g., a few nanoseconds). The combination of a low repetition rate and a short pulse duration results in a fluence delivered to a wafer under inspection that far exceeds the damage limit of materials used to construct the wafer (e.g., SiO2, Si, metals, and resist materials).
In some other examples, illumination light may be provided by an incoherent light source pumped by a laser (e.g., laser sustained plasma). Laser sustained plasmas are produced in high pressure bulbs surrounded by a working gas at lower temperature than the laser plasma. While substantial radiance improvements are obtained with laser sustained plasmas, the temperature of these plasmas is generally limited by the photophysical and kinetic processes within these lamps. Pure atomic and ionic emission in these plasmas is generally confined to wavelengths longer than 200 nm when using either continuous wavelength or pulsed pump sources. Excimer emission can be arranged in laser sustained plasmas for wavelength emission at 171 nm (e.g., xenon excimer emission), but these sources are typically narrow band, limited in power, and limited in radiance. Excimer emission at 171 nanometers optimizes at low pressures (e.g., 3 bar and below), and the power of 171 nm emission is greatly diminished at higher pressures needed for high radiance. As a consequence, a simple gas mixture in a high pressure bulb is only able to sustain wavelength coverage above 200 nm with sufficient radiance and average power to support high throughput, high resolution BF wafer inspection.
Development efforts in the area of extreme ultraviolet (EUV) lithography are focused on light sources that emit narrowband radiation centered at 13 nanometers at high power levels (e.g., 210 watts of average power at the intermediate focus of the illuminator). Light sources for EUV lithography have been developed using a laser droplet plasma architecture. For example, xenon, tin, an lithium droplet targets operating at pulse repetition frequencies of 100 kHz and higher are pumped by CO2 coherent sources. The realized light is high power (e.g., 210 watts of average power at the intermediate focus of the illuminator is the goal for lithography tools at 13 nanometers). However, the materials that comprise a semiconductor wafer exhibit practically no reflectivity to narrowband light at 13 nanometers.
Shorter wavelength illumination sources with the required radiance and average power for BF inspection applications are required. Preferably, such a source should be continuous wavelength or close to continuous wavelength to avoid damage to illuminated specimens. Furthermore, manufacturable objectives and sensors compatible with such sources are required to realize a viable BF inspection system.